1. Field of the Invention
The present invention relates to a transistor power amplifier for transmitting systems, particularly high frequency systems, having two parallel-connected signal paths which are fed on the input side with an input signal by way of a first branch connection and are again combined on the output side by way of a second branch connection.
2. Description of the Prior Art
Power amplifiers having power outputs from between several 100 watts to several 1000 watts are required, for example, for transmitter stages in high frequency transmission systems, such as relay stations, among other applications. In short wave transmitters in the frequency range of between 1.5 and 30 MHz, the field of application of point-to-point connections by way of medium and large distances are of interest, whereby high requirements are made with respect to the linearity of amplification, to such an extent that, according to CCIR, for example, intermodulation values of 35 dB are required for specific instances of application.
In contrast to tube technology, the power required can be delivered with transistor transmitters in a single amplifier stage in the lowest values, at best, due to the limitation of permissible temperatures and heat resistances of transistors. One manner of obtaining a higher power output, therefore, most often involved a parallel connection of a plurality of push-pull stages, such as is apparent from the block circuit diagram illustrated in FIG. 1, for example. An input signal E is first separated into two signal paths by way of a first branch connection 1, such as a two-four wire coupling, and each of these paths are further separated into a total of four signal paths by way of two additional branch connections 2 and 3. An amplifier stage 4, 5, 6, 7 is provided in a respective path. At the output side, the amplifiers are interconnected in pairs, respectively, by way of two branch connections 8, 9 and finally interconnected by way of another branch connection 10 to provide an output A. However, in addition to the insufficient power output in the case of transistor amplifiers, there is also the problem of insufficient linearity, so that the required intermodulation intervals can most often not be maintained. An improvement in the linearity by means of a sub-operation cannot be achieved for Class B operation. Similarly, inverse feedback measures are ruled out, practically speaking, because of the low cut-off frequency of broad band amplifiers and the variable terminating impedances.
An additional prior art possibility for improving the linearity of high power broad band amplifiers results from the application of the so-called "feed-forward" method, which has also become known as "forward coupling". In this connection, one may refer to United States Letters Patent 3,649,927.
FIG. 2 illustrates in block diagram form, an amplifier which functions according to the feed forward principle. The input signal E is uniformly separated into two parallel signal paths I and II by way of a branching network, for example a branch connection 11. The input signal E transmitted in the signal path I first passes through a primary amplifier HV having an amplification G1 and a group or envelope delay .tau.1. The output of the amplifier HV is subject to an error component characterized by non-linearities. A fraction of the output signal of the amplifier HV is provided by way of a directional coupler RK, for example. The fraction tapped is attenuated by the factor 1/G1 and compared with the output signal transmitted in the signal path II which has been delayed by an amount .tau.1 with the aid of a cable, for example. By subtracting both signals, the pure error component .epsilon. remains, which error component is raised to the level of the main amplifier output signal in a correction amplifier KV connected in the signal path II. An additional delay member simulates the delay .tau.2 of the correction amplifier KV. An additional branch connection 12 is provided at the output of the amplifier arrangement in which the delayed output signal (with the error component .epsilon.) of the primary amplifier HV in the signal path I and the error component .epsilon., with a negative sign, appearing at the end of the signal path II, are combined in such a fashion that both error components cancel one another.